1. Field of the Invention
This invention relates to a double nut of the type which comprises a tightening nut body for clamping articles such as a pair of plates between itself and an enlarged head of a bolt and a locking nut body for frictionally preventing unexpected loosening of the tightening nut body, and more particularly to such a double nut which is improved to provide an exactly predetermined tightening torque.
2. Description of the Prior Art
In clamping for example a pair of plates having aligned perforations with use of a bolt and a nut, a threaded shank of the bolt is inserted through the perforations, and the nut is screwed on the bolt shank and tightened up properly. Such a clamping technique, however, has a disadvantage of resulting in variations in the clamping or tightening force achieved, often causing excessive or insufficient clamping. Another disadvantage of this technique is unexpected loosening of the nut or bolt during subsequent use of the clamped plates.
The problem of unexpected loosening is eliminated or alleviated by an arrangement which is disclosed in Japanese Utility Model Publication No. 50-36123 and which is illustrated in FIG. 11 of the accompanying drawings for the convenience of explanation.
Referring now to FIG. 11, there is shown a combination of a tightening nut 100 having a threaded axial bore 100a and a locking nut 101 similarly having a threaded axial bore 101a. The tightening nut 100 is formed with a recess 102 defined by a conical internal surface and positioned coaxially with the axial bore 100a. The locking nut 101, on the other hand, is formed with an axial projection 103 having a conical external surface and positioned eccentrically with the axial bore 101a. The axial projection 103 is insertable into the recess 102.
In use, a threaded shank 104a of a bolt 104 having an enlarged head 104b is inserted through aligned perforations 105a of a pair of plates 105 to be clamped, and the tightening nut 100 is screwed onto the bolt shank 104a and tightened up properly so that the plates 105 are clamped together at a certain force between the tightening nut 100 and the bolt head 104. Subsequently, the locking nut 101 is screwed on the bolt shank 104a until the conical external surface of the axial projection 103 comes into frictional and wedging contact with the conical internal surface of the recess 102, so that the tightening nut 100 is frictionally prevented from unexpected loosening.
Because of the eccentricity of the axial projection 103, the wedging contact between the conical external surface of the projection 103 and the conical internal surface of the recess 102 results in occurrence of opposite lateral forces as indicated by arrows X, Y in FIG. 11, whereby the two nuts 100, 101 are laterally pressed against the bolt shank 104a. Naturally, such lateral forces also contributes to the prevention of unexpected loosening of the nuts.
The arrangement of FIG. 11, however, still fails to eliminate a disadvantage of excessively or insufficiently clamping the plates 105, which can result in damaging the plates or unacceptable play among the parts involved. Further, this arrangement provides a new drawback of requiring separate screwing of the two nuts 100, 101, thus reducing operability.
In an attempt to prevent non-uniform clamping and to improve operability, Japanese Patent Publication No. 43-23569 proposes such a double nut as illustrated in FIGS. 12 and 13. More specifically, the double nut comprises a tightening nut body 200 having a threaded axial bore 200a and a locking nut body 201 also having a threaded axial bore 201a and joined to the tightening nut body 200 by means of a thin-walled, axial connecting web 202.
In operation, a threaded shank 203a of a bolt 203 having an enlarged head 203b is inserted through aligned perforations 204a of a pair of plates 204, and the double nut is screwed on the bolt shank 203a and tightened up by a wrench (not shown) engaged with the locking nut body 201. Upon further turning the locking nut 201, the connecting web 202 breaks under a certain circumferential shearing force, so that the locking nut body 200 comes into frictional contact with the tightening nut body 201 at the broken connecting web 202 to prevent unexpected loosening of the double nut. The plates 204 are clamped substantially at a tightening torque at which the connecting web 202 breaks. Thus, by altering the wall thickness of the connecting web 202, it is possible to adjust the tightening torque achieved by the double nut.
In reality, however, because the axial connecting web 202 is subjected to axial compression at the time of breaking under a circumferential shearing force, the web 202 actually breaks at a tightening torque which is higher by about 20 to 30% than theoretically expected from the wall thickness of the web 202. Thus, it is impossible to exactly predetermine a tightening torque at which the web 202 breaks. For this reason such a double nut has never been put to practical use.